Can anyone tell me the name of this book?!

I read a book quite a while back and I can't remember the author or the name of it, I've searched different places but can't find anything. It's a murder story and it's basically one person is taking two people, drugging them and then the people wake up and one has to kill the other in order to survive. The only thing I really remember is that every time two victims went missing, they woke up and the only way to survive was to kill the other - basically a case of 'kill or be killed.' One set of victims woke up in a huge tank and one had to shoot the other first then climb out of the tank. I also recall the method of the person drugging the two victims and being able to drag them along was by using a harness used to drag deer along when they had been killed. If anyone could tell me the name or author I'd be extremely grateful! X